1. Field of the Invention
This invention relates to linear scale type displacement measuring instrument, and more particularly, to improvements in a linear scale type displacement measuring instrument suitable for use in an optical type displacement measuring instrument including a main scale and an index scale, which are formed of a transparent material such as glass.
2. Description of the Prior Art
There has heretofore been known a linear scale type optical displacement measuring instrument including:
a hollow elongate case fixed to one of relatively movable members;
a main scale secured to the inner surface of the case in the longitudinal direction thereof;
a carriage connected to the other of the relatively movable members and movable along the main scale in the longitudinal direction thereof;
index scales held by the carriage as opposed to the main scale; and
a detector for converting a displacement value of relative movement between the main scale and the index scales into an electric signal.
Since the overall accuracy of an order of about one micro meter is required in the displacement measuring instrument of the type described, the requirements to respective elements and assembled structure thereof are quite strict. Even if the respective elements are on the same level, there are such special circumstances that a difference between the assembled structures is directly connected to the quality level of the accuracy as it is.
As one of special circumstances peculiar to the displacement measuring instrument of the type described, there is a problem that the displacement measuring instrument reliably assembled under a predetermined construction cannot be mounted in as assembled state to a machine or the like. Namely, when mounted to the machine, the displacement measuring instrument is subjected to an external force, with the result that a warp is caused to the main scale which has been straightenedly adjusted. This warp generates a difference between an actual movement value of the carriage connected to the other of the relatively movable members and a displacement information read out of the main scale having the warp, thus presenting the disadvantage of not securing the detecting accuracy.
In order to reduce an error due to the warp, there has heretofore been taken such a measure that, from a viewpoint of that the warp of the main scale generated during mounting is unavoidable with some degree in difference being admitted, the main scale is assumed to be a simple beam freely held in the air and applied at opposite ends thereof with bending moments. For example, in Japanese Patent Publications Nos. 27803/81, 5441/82, Japanese Patent Laid-Open No. 9007/83 and the like, there are disclosed constructions in which connection of the carriage is made within the neutral plane of the main scale itself or at the center of a graduated surface. Namely, in these examples of the prior art, as for the thicknesswise direction of the main scale for example, the intermediate portion in the thicknesswise direction of the main scale is assumed to be the neutral plane, and, under the prerequisite that an extension occurs on one side of the neutral plane and a shrinkage occurs on the other side, the carriage is connected in a manner to be rotatable relative to the main scale within the neutral plane.
However, all of these examples of the prior art have been unsatisfactory in effects, and, along with the highly improved accuracy in recent years, necessity has been voiced for the improvements in this respect. Namely, the warp of the main scale actually occurring during mounting is observed along with a distortion of the case, to which the main scale is secured. In consequence, the warp of the main scale behaves as commensurate to the curvature of the case, depending on the position of the main scale in the case. Namely, if consideration is given to that the warp of the main scale conforms itself to such a characteristics as in a composite beam formed of materials different from each other, then these examples of the prior art have been quite imperfect.